This pertains to communication systems and, more particularly, to allocation by a transmitter of transmission power to a plurality of channels.
One characteristic of current-day mobile communication systems is that a base station provides service to a large number of mobile units that are permitted to travel while communicating. Another characteristic of such communication systems is that the capacities of the individual channels that are used for transmitting to the mobile units vary with time. Yet another characteristic is that the transmission power is limited. The transmitter of a base station is thus faced with a situation where a plurality of channels of different capacities need to serve a plurality of mobile units that expect data flow of respective rates, over respective bandwidths, with respective tolerances to errors, all in an environment where the qualities of the channels are not known with precision.
The qualities of the channels are not known with precision because the best knowledge that the transmitter has is obtained from feedback, such as Channel Quality Indicator (CQI) messages, that is sent by the mobile units. The measurement of the channel quality, the coarse CQI encoding, the transmission of the CQI messages to the base station, and the decoding of those messages all take time, in addition to containing significant quantization errors and, consequently, the best information that a base station has of channel quality is a coarse estimate of past performance. This information is, therefore, of questionable value in fine allocation of transmitter power.
The challenge is to assign transmission power to channels in the most effective manner, taking account of the different performance levels that are required, or acceptable, in an environment where only the aforementioned coarse estimates of the qualities of the channels are known.
The prior art handles this uncertainty in the channel quality knowledge by employing a fade margin, which adds several dBs to the transmit level to insure successful communication in the event the actual channel's conditions are worse than what the CQI messages indicate.
The problem with the prior art approaches is the inability to optimize transmitter/receiver operation to the case at hand when firstly we are dealing with limited radio resources (limited power, limited bandwidth), and secondly the channel is known with a statistical error. Consequently, prior art solutions end up either with wasted power (too big margins) or excessive error-rate if margins are insufficient. In both cases the effective communication rate is less than can be obtained if optimal solution is used.